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Europe experienced an historic heat wave in the summer of 2003. This image shows the differences in daytime land surface temperatures collected in 2003 and 2001 by the MODIS instrument on NASA’s Terra satellite. A blanket of deep red across France shows where temperatures were 18°F (10°C) hotter in 2003 than in 2001. White areas show where temperatures were similar, and blue shows where temperatures were cooler in 2003 than 2001. (Image courtesy NASA Earth Observatory.)

May 20, 2010 | Heat waves may not evoke the same sense of urgency as earthquakes, floods, or severe weather, but they’ve caused more deaths over the past fifteen years than all other extreme weather events combined. By some estimates, between 40,000 to 70,000 people were felled by Europe’s 2003 heat wave alone.

A study led by NCAR visiting scientist Erich Fischer (based at Switzerland’s ETH Zurich) analyzes a set of high-resolution regional climate simulations to project where heat-wave-related health risks will increase in Europe as Earth’s climate warms. The research, published in Nature Geoscience on May 16, uncovered a geographically consistent pattern among the models, despite substantial differences in the projected magnitude of the changes.

Using six different climate models, the scientists focused on three climatic risk factors that contribute to mortality during heat waves: the duration of a heat wave; warm nights (in combination with extremely hot days), during which the body can’t recover from daytime heat; and relative humidity, since the body’s evaporative cooling becomes less efficient in high humidity.

The model results were consistent in projecting more days with high health risks for the time frames of 2021–2050 and 2071–2100concentrated in southern European river basins and along the Mediterranean coast. Notably, the areas with the greatest risk also have high population densities, such as the major urban areas of Rome, Naples, Milan, Athens, and Marseille. Despite a slight reduction in relative humidity, days with dangerous health conditions become more frequent in these areas due to heat waves that are more intense, frequent, and longer-lasting.

The authors note that air pollution and urban heat island effects aren’t accounted for, yet these may further amplify health risks. They also stress that, particularly for young and healthy persons, it is possible to adapt to the new conditions—up to a point.